The disclosure relates to a thermal printer, and more particularly to a thermal printer that controls heat buildup in a thermal head thereof.
A thermal printer generally has a plurality of heating elements which are arrayed in at least one row on a thermal head. Drive pulses are selectively applied to the heating elements to generate heat for printing. The heat accumulates or builds up in the thermal head as printing proceeds, and print characteristics or print performance of the thermal printer can be adversely affected by this heat buildup in the thermal head. For example, printing high-density patterns continuously under hot conditions can lead to problems such as fattened or illegible letters (or patterns) due to the heat buildup. Some thermal printers have been proposed to overcome such problem.
For example, as disclosed in Japanese patent-application publication No. 2001-270144, a thermal printer calculates heat buildup data that indicates a heat buildup state in each heat buildup layer (a glaze layer, a ceramic plate, and an aluminum plate) of its thermal head, and multiplies the heat buildup data by predetermined coefficients, thereby obtaining heat buildup correction data. The thermal printer then obtains heat generation data based on the heat buildup correction data, and controls driving of heating elements based on the heat generation data. The aluminum plate is provided with a head temperature sensor that measures temperature of the thermal head. A coefficient to be used to obtain the heat buildup correction data for the aluminum plate is determined by an equation that includes a head voltage as a parameter.
Another thermal printer applies, in addition to main drive pulses, sub-pulses (subsidiary pulses) having a short ON time in order to compensate for any energy insufficiency at the start of printing and to prevent print blurring. For example, Japanese patent-application publication No. 7-108701 discloses a thermal printer that applies two types of sub-pulse to the thermal head based on a dot print state at the current dot and at the dot before the current dot. The thermal printer also adjusts the ON time by changing the width of the main pulses.
It is known that a pulse width applied to the heating elements are controlled in consideration of load fluctuations, when a voltage of a power source is not constant (such as when a stabilized power source like a DC/DC converter is not used). This control method is called unstable control. In the unstable control, when a dot count (a number of dots) to be printed is high, a current is also high and thus a voltage drop is large. On the other hand, when the dot count to be printed is low, the current is also low and thus the voltage drop is small. For that reason, there arises a time difference for a temperature rise of the heating elements between the high voltage state and the low voltage state. A thermal printer disclosed in Japanese patent-application publication No. 8-300713 reads out the voltage every time Δt, determines an OFF time for application pulses based on the read voltage, and repeats ON and OFF of the pulses based on the OFF time. In this way, the thermal printer provides a stable heat generation of the thermal head.
A thermal printer disclosed in Japanese patent-application publication No. 2001-191574 calculates a heat buildup level for each heating element based on a number of drive pulses which have been applied over a plurality of times. The thermal printer applies, to each heating element, a number of drive pulses that corresponds to the heat buildup level. In this way, size and density of a dot pattern to be recorded by each heating element is made uniform.